1. Field of the Invention
The present invention generally relates to the art of microelectronics, and more specifically to a multilayer ceramic tape structure having sensor elements formed therein.
2. Description of the Related Art
Techniques have been developed for integrating sensors to measure force or pressure, acceleration, temperature, position or displacement, ion (ph) and gas concentrations, magnetic field strength, radiation levels, etc., into a monolithic structure with signal processing electronics. An integrated device can be made much smaller, lighter, and cheaper than a package including a separate sensor and its associated components, and is potentially more reliable.
Such integrated sensor packages are being used extensively, for example, in the automotive industry, in applications involving electronic controls to optimize fuel economy and engine operation, meet emission control requirements, and provide more comfortable and/or safe driving characteristics. Assemblies using such sensors include antilocking and/or antiskid braking systems, positive traction systems, suspension adjustment systems, and the like.
The two main prior art types of integration include micromachining of silicon, and thick film processing. A general description of these technologies is found in an article entitled "New Advances in Sensor Technology", by L. Teschler, Machine Design, Dec. 6, 1984, pp. 118-124.
Micromachining uses the same photolithographic and chemical processes as conventional integrated circuit fabrication. Doping dependent anisotropic chemical etching is a major micromachining method. However, devices have been fabricated using dry etching techniques utilizing plasma and reactive ion beams. Possible structural permutations include grooves, free-standing pillars, cantilevered beams, membranes of various thicknesses with and without integral pores, microbridges, and various shapes of holes. A detailed discussion of sensor structures fabricated by micromachining of silicon is found in an article entitled "Silicon Micromechanical Devices", by J. Angell et.al, Scientific American, Apr. 1983, pp. 4455. An example of a multi-dimensional accelerometer formed by micromachining of silicon is found in U.S. Pat. No. 4,809,552, issued Mar. 7, 1989, entitled "MULTIDIRECTIONAL FORCE-SENSING TRANSDUCER", to G. Johnson.
Although advantageous in many respects, micromachining of silicon is a relatively complex and expensive process, and is limited in the configurations of sensor shapes that can be formed.
Fabrication of multilayer electronic structures for hybrid microcircuit technology and other applications includes the thick film process referenced above wherein individual conductor and dielectric compositions in paste form are sequentially deposited on insulating substrates and then fired, one layer of material at a time, in order to build up a thick film, multilayer circuit. Sensors responsive to pressure, stress, displacement, etc., have been fabricated using the thick film process by exploiting the piezoresistive effect in thick film resistors. Such resistors are formed on mechanical sensor elements such as cantilevers and diaphragms, and transduce mechanical strains into electrical signals. An article describing this technology is found in "THICK-FILM PRESSURE SENSORS: PERFORMANCES AND PRACTICAL APPLICATIONS", by R. Dell'Acqua et.al, Third European Hybrid Microelectronics Conference Proceedings, Avignon, 1981.
The major problem inherent in the thick film process is that thickness control is difficult in the formation and machining of fired ceramic layers. This imposes a serious limitation on the accuracy attainable with sensors formed by this process.
An improved method for the fabrication of hybrid microcircuits which forms a basis for the present invention is the cofired ceramic process. This technology utilizes dielectric material formed into sheets having alumina as a main component. These insulating sheets are then either metallized to make a ground plane, signal plane, bonding plane, or the like, or they are formed with via holes and back filled with metallization to form interconnect layers. Individual sheets of tape are then stacked on each other, laminated together using a predetermined temperature and pressure, and then fired at a desired elevated temperature at which the material fuses or sinters. Where alumina is chosen for the insulating material, tungsten, molybdenum or molymanganese is typically used for metallization, and the part is fired to about 1,600.degree. C. in an H.sub.2 reducing atmosphere.
The undesirable high processing temperature and requisite H.sub.2 atmosphere of the refractory metals has led to the development of Low-Temperature-Cofired-Ceramic (LTCC) tape. LTCCs are under development and/or commercially available from a number of manufacturers including ElectroScience Laboratories, Inc., of Prussia, Pa., EMCA, of Montgomeryville, Pa., and FERRO, of Santa Barbara, Calif. A preferred LTCC material, which is known in the art as "green tape", is commercially available from the DuPont under the product designation #851AT. The tape contains a material formulation which can be a mixture of glass and ceramic fillers which sinter at about 850.degree. C., and exhibits thermal expansion similar to alumina.
The low-temperature processing permits the use of air fired resistors and precious metal thick film conductors such as gold, silver, or their alloys. In the typical high-temperature process, screen-printed resistors cannot be used and only refractory metal pastes are used as conductors.
A discussion of thick film technology, and high and low temperature cofired ceramic tape technology, is found in "DEVELOPMENT OF A LOW TEMPERATURE COFIRED MULTILAYER CERAMIC TECHNOLOGY", by William Vitriol et.al, ISHM Proceedings 1983, pp. 593-598.
One disadvantage of the cofired ceramic approach is that the dielectric film or tape will undergo shrinkage of as much as 20% in each of the X, Y, and Z directions. This shrinkage results in a dimensional uncertainty in the fired part of typically .about.1%, which may be unacceptable in the fabrication of certain types of hybrid microcircuits.
Another multilayer circuit board fabrication technology which obviates the shrinkage problem inherent in the ceramic cofired tape process is disclosed in U.S. Pat. No. 4,645,552, issued Feb. 24, 1987, entitled "PROCESS FOR FABRICATING DIMENSIONALLY STABLE INTERCONNECT BOARDS", to William Vitriol et al. This process may be described as a "transfer-tape" method, and is performed by providing a generally rigid, conductive substrate, or an insulative substrate on which a conductive circuit pattern is formed, and then transferring and firing a glass-ceramic tape layer to the surface of the substrate. This tape layer provides electrical isolation between the substrate and electrical conductors or electronic components which are subsequently bonded to or mounted on the top surface of the glass-ceramic tape layer. By providing vertical electrical interconnects by means of vias formed in the tape layer prior to firing the tape layer directly on the substrate, X and Y lateral dimensional stability of the tape material is maintained. The next conductor layer in this vertical interconnect process is then screen printed on the fired tape dielectric and itself fired. This process is repeated until the hybrid circuit is built up to a desired vertical, multilayer interconnect level. As an alternative process to individually firing conductor and dielectric layers, the complete structure or portions thereof can be simultaneously fired as disclosed in the above referenced patent to Vitriol. By replacing a screen printed dielectric layer build-up process with a pre-punched dielectric tape layer, the transfer tape process retains the primary advantages of the thick film process, while gaining many advantages of the cofired ceramic process.